Various spectral photometric techniques have been developed for the noninvasive monitoring of blood constituent concentrations. In such systems, light of multiple wavelengths is used to illuminate a thin tissue portion of a person, such as a fingertip or earlobe. A spectrum analysis of light transmitted through or reflected from the tissue portion is used to measure the light absorption characteristics of blood flowing through the tissue portion. Utilizing calibration data, the concentration of various blood constituents is then derived from known light absorption characteristics of these blood constituents. In one spectral photometric methodology, the absolute optical spectrum of light received from the tissue portion is measured. In a differential spectral photometric methodology, blood constituent concentrations are derived from photoplethysmograph data that is responsive to blood volume changes. Pulse oximetry systems, which use the latter methodology to monitor hemoglobin constituents, have been particularly successful in becoming the standard of care for patient oxygen saturation monitoring.